Engine Overhaul - Major or "Top"
Field Information Number 308
Use of Unleaded Automobile Gasoline in Freshly Overhauled Engines
The purpose of this bulletin is to explain the procedure for the use of unleaded automobile gasoline in new or freshly overhauled engines where the valves have been refaced. There have been reports from the field of valve sticking or actual valve/valve seat damage when unleaded automobile gasoline was used in some freshly overhauled engines that have previously used automobile gasoline successfully for long periods. Subsequent valve problems occurred in as little as 10 hours of engine operation.
Small engines from the 1940s were designed for fuels with octane ratings of 63 to 73 octane. These fuels contained no lead at that time. There were valve sticking problems in some engines which were resolved by the manufacturer issuing service instructions to change the valve seat angle and to replace soft valve seats with hard seats. This had been done so many years ago that there is little likelihood of an engine still in existence which has the original soft seats and old valve seat angles. Thus aviation gasoline, with its very high lead content (approximately 2.0 grams/gallon) compared to the need for these engines, had caused many problems as outlined in AC 91.33. Note that grade 80 aviation gasoline for which these engines were initially approved, has a maximum allowable lead content of approximately 0.5 grams/gallon and may be supplied with zero lead content.
Engine manufacturers, in order to minimize valve problems caused by the excessive lead content of 100 low lead aviation gasoline, have increased valve stem diameters and added hardened inserts for the valve seats. Both of these changes favored the use of a gasoline with a very low lead content also, and as pointed out above, showed normal wear characteristics in the EAA's 500-hour flight test program.
As indicated in the June 1985 issue of EAA's SPORT AVIATION magazine, a Continental Motors Special Service Bulletin M46-2, dated November 25, 1946, addressed this situation. An investigation by Continental Motors revealed that the "absence of lead from the fuel has resulted in the 'picking up' or 'welding' of the valve seat to the valve." (Remember, at that time, high quality gasoline was "white gasoline" - with no lead content.)
The special bulletin goes on to say that "there is nothing wrong with either the gasoline or valve materials, but that the two will not work together except after a protective coating is deposited on the valve and seat contact surfaces by use of leaded fuel for the first two or three hours of life of the new engine or an engine in which the valves have been refaced, after the initial 'lead treatment,' however, no valve trouble should be encountered and operation should be better with the 73 or 80 octane clear gasoline than with the war and prewar leaded fuels." Since that time, TCM has changed valve and seat angles for further improvement.
Although this procedure was not used during our flight test program and none of the above mentioned valve problems were experienced, we recommend operating on leaded fuel immediately after engine service involving the valves.
EAA members report that their experience indicates that 10 hours of operation on leaded fuel (100LL aviation gasoline, not "leaded" regular automobile gasoline, which now has no lead content) is a more conservative program.
Experience has shown greater extended life, and more time between overhauls, with the use of unleaded automobile gasoline. EAA had more than 500 hours of flight test time on each of the following aircraft when EAA's STC's were issued: Cessna 150 with a Continental engine, Cessna 182 with a Continental engine, Piper Cherokee PA-28-140 with a Lycoming engine, and a Cessna 172 with a Lycoming engine. A Cessna 172 in pipeline patrol satisfactorily flew a supervised 1,200 hours observed by the FAA. All aircraft performed very satisfactorily in actual flight tests for EAA and the FAA. Engine teardown inspection by an independent engine shop and witnessed by FAA representatives indicated no adverse wear problems with the use of unleaded automobile gasoline.